This invention relates to process for producing an integral composite engineered panel product having at least one side which is substantially smooth, and more particularly, a durable, engineered wood composite product formed of lignocellulosic strands or wafers (xe2x80x9cOSBxe2x80x9d) at least one outer surface of which remains substantially smooth despite exposure to wet and/or humid weather conditions.
Products such as fiberboard and particleboard have been found to be acceptable alternatives in most cases to natural wood paneling, sheathing and decking lumber. Fiberboard and particleboard are produced from wood particles bonded together by an adhesive, the adhesive being selected according to the intended use of and the properties desired for the lumber. Often times, the adhesive is combined with other additives to impart additional properties to the lumber. Additives can include fire retardants, insect repellants, moisture resistants, fungus resistants and color dyes. A significant advantage of fiberboard and particleboard lumber products is that they have many of the properties of plywood, but can be made from lower grade wood species and waste from other wood product production, and can be formed into lumber in lengths and widths independent of size of the harvested timber.
A major reason for increased presence in the marketplace of the above-described product alternatives to natural solid wood lumber is that these materials exhibit properties like those of the equivalent natural solid wood lumber, especially, the properties of retaining strength, durability, stability and finish under exposure to expected environmental and use conditions. A class of alternative products are multilayer oriented wood strand particleboards, particularly those with a layer-to-layer oriented strand pattern, such as OSB. Oriented, multilayer wood strand boards are composed of several layers of thin wood strands, which are wood particles having a length which is several times greater than their width. These strands are formed by slicing larger wood pieces so that the fiber elements in the strands are substantially parallel to the strand length. The strands in each layer are positioned relative to each other with their length in substantial parallel orientation and extending in a direction approaching a line which is parallel to one edge of the layer. The layers are positioned relative to each other with the oriented strands of adjacent layers perpendicular, forming a layer-to-layer cross-oriented strand pattern. Oriented, multilayer wood strand boards of the above-described type are described in detail in the following: U.S. Patents: U.S. Pat. No. 3,164,511, U.S. Pat. No. 4,364,984, U.S. Pat. No. 5,435,976, U.S. Pat. No. 5,470,631, U.S. Pat. No. 5,525,394, and U.S. Pat. No. 5,718,786, all of which are incorporated herein by reference.
Certain oriented board products can be made from flakes that are created from debarked round logs by placing the edge of a cutting knife parallel to a length of the log and the slicing thin flakes from the log. The cut flakes are subjected to forces that break the flakes into strands having a length parallel to the grain of the wood several times the width of the strand. The strands can be oriented on the board forming machine with the strands predominantly oriented in a single, e.g., cross-machine direction in one, e.g., core layer and predominantly oriented in the generally perpendicular (machine) direction in adjacent layers. The various layers are bonded together by natural or synthetic resin(s) under heat and pressure to make the finished product.
Oriented, multilayer wood strand boards of the above-described type are produced with bending, tensile strengths and face strengths comparable to those of commercial softwood plywood. However, the elongated wood strands forming oriented wood strand boards typically have an irregular contour and leave voids, as a result, form a rough surface finish. Even sanding the board surface may not result in producing smooth surface finishes on oriented wood strand boards comparable to those routinely produced on natural solid wood boards. To form a smooth surface on oriented wood strand boards, it has been suggested to place a layer of wood particle fines on the finish surface of the board. A multilayer wood particleboard can be manufactured, for example, having a core portion of at least two layers composed substantially of adhesively bonded wood strands, and at least one layer composed substantially of adhesively bonded wood fines bonded to cover an outer layer of the core portion to form a finished surface. The wood strands are distributed in each core layer with their length in substantial parallel orientation in a preferred direction. Furthermore, the adjacent layers of the core are positioned relative to each other with the oriented strands of each layer perpendicular to the oriented strands of the adjacent layer, to thereby form a layer-to-layer oriented strand pattern multilayer core structure. In some cases, one or more core layers may be randomly oriented.
A smooth outer surface can be created on board products by adding a sufficient layer of fine wood particles. In U.S. Pat. No. 4,364,984, for instance, the layer of fines is described as being composed of irregularly contoured wood particles of various sizes having a width and thickness considerably less than average width of the wood strands forming the core layers. This prior art invention requires that the fines be xe2x80x9cgradedxe2x80x9d for uniform size, and that the distribution of these graded fines be specific. More specifically, the fines after being graded to form particle fractions of substantially uniform smaller or larger size, are formed into a fines layer in which the smallest sized particle fraction is located at a first major surface of the fines layer and largest sized particle fraction is located at a second major surface of the fines layer. The size of the fines particles located therebetween are of a graduated size, the graduated size of the particles ranging from smaller to larger from the first to the second major surface of the fines layer.
These fines typically are said to be defined as wood particles having a width and thickness less than 0.50 mm. However, with respect to the invention of the ""984 patent, the term xe2x80x9cfinesxe2x80x9d is used in the broader, more generically relative sense to define wood particles having an average width and thickness several times smaller than the average width of the wood strands used to form the associated core layers. These fines wood particles are distributed in an unoriented graduated size pattern from the outer surface to inner surface of the core-covering surface layer, with the progressively smaller or finer wood particles being distributed closer to the outer surface of the surface layer. With the fines distributed in this unoriented, graduated size pattern, the largest fines wood particles are in contact with the surface of the covered core layer of the multilayer board to provide more contacting surface area for bonding the surface layer of fines to the covered core layer. By increasing the surface area of contact at the interface between the covered core layer and surface layer, it is concluded that a much stronger bond is capable of being formed between the layers. By avoiding orientation of the fines wood particles, the particles are said to interlink to aid the adhesive in the creation of a strong bond between the fines forming the surface layer. By combining the unoriented, graduated size pattern of distribution of fines in the surface layers with the layer-to-layer oriented pattern of distribution of strands in the core layers of the multilayer wood particleboard structure it is concluded in U.S. Pat. No. 4,364,984 that the strength, durability, stability and finish properties typifying natural solid wood lumber can be produced in lumber made from such multilayer wood strandboard structures.
U.S. Pat. No. 4,068,991 discloses a particleboard, e.g., chipboard product that includes a continuous particle size gradient between a coarser particle core and a finer particle surface layer. Since the fines are discrete wood fibers, they do not consolidate into a tight surface, but rather, retain susceptibility to the ready entry of water and do not holdout paint to a satisfactory degree. The particle size gradient transition from one particle size to another can be continuous or step-wise.
Telegraphing is the protrusion through a veneer, plastic or paper outer covering, or a painted outer surface of one or more layers of wood, which make up the composite panel. One attempt to prevent telegraphing is described in U.S. Pat. No. 3,098,781. The ""781 patent discloses a particleboard product made entirely from fine wood particles of graduated size. The product formed in the ""781 patent has no distinct layered separation. The ""781 patent produced a particleboard which supposedly has the advantage of accepting a covering of veneer, paper or plastic sheets without telegraphing the relatively irregular surface of the underlying particleboard. The covering is joined to the particleboard, after formation of the particleboard, in a subsequent manufacturing operation.
Products such as cedar-like siding have been sold which include a resin bonded outer covering of embossed paper adhered to one surface of an engineered wood product, such as OSB. The paper covering is embossed. Telegraphing is not a problem with respect herein since the strands or flakes which comprise this product are not readily visible through the covering if it is embossed.
One of the problems associated with the application of fines onto an OSB baseboard is achieving a strong bond at the interface between the OSB and the fines which is capable of resisting weathering. The ""984 patent suggests that a strong bond can be achieved at the interface between an OSB product and a fines particle layer by manufacturing the OSB with the largest OSB flakes at the interface, and applying the layer of fines particles such that the longest fines are disposed at the interface. Similarly, U.S. Pat. No. 4,361,612 discloses that shorter fibers in the surface of an OSB product will degrade the bending strength of an OSB product. Further, the ""612 patent teaches that a laminated wood product including a flakeboard core laminated to a wood veneer, a wet-process hardboard or a wet-process fiberboard covering, as in typical plywood practice, may need a core finishing operation on a drum sander to achieve a core surface capable of good bonding to the covering.
U.S. Pat. No. 3,308,013 suggests that a water-laid fiber sheet containing resin and having a high basis weight of dry fiber can be employed to mask defects in plywood, particleboard, and the like. These heavy papers have been used to produce medium density overlain plywood that has found application in road signs where the smooth surface accepts lettering and reflective laminates. High cost, limited embossability, poor weathering, and poor adhesion of coatings preclude the use of this product in siding applications.
It has heretofore been generally accepted by those skilled in the art that an OSB baseboard and a fiberboard covering will not form a good bond at their interface and that the differential in dimensional and elastic properties of the fiberboard and OSB materials will result in delamination because of moisture cycling due to weather conditions.
All of the U.S. patents cited above are incorporated herein by reference in their entirety.
The present invention relates to multi-layer integral, composite engineered panel having a durable, smooth outer face on at least one of its major surfaces, which remains substantially smooth despite exposure to wet or humid conditions. In this way, lower cost wood products, such as OSB, can be upgraded by having a smooth outer surface appearance in uses, which in the past have been the domain of lumber based virgin wood products. An example of a major market for this new type of product would be xe2x80x9cengineered wall panelsxe2x80x9d.
This invention overcomes problems in the manufacture of the above-described product, which are smooth, paintable and will not telegraph. As previously discussed, telegraphing is a condition where the texture of a multi-layer product is revealed through the smooth outer covering surface upon exposure to water or high humidity.
Panels of the type described herein such as OSB-based panels, on formation, have rough surfaces. These surfaces become even more irregular upon exposure to moisture due to uneven swelling of the strands or wafers which make up the panel structure. Such products are therefore unsuitable for use where, for example, a smooth exterior face is required. For instance, paintable surfaces, which are required to be smooth, include flat exterior panels, interior wall panels, furniture, etc.
Sanding greatly improves the initial surface smoothness, but individual strands or wafers, which form products similar to the panels of the present invention, will readily telegraph with exposure to water or high humidity. Furthermore, the major surfaces of these prior art products are not readily paintable due to both grain raise at time of painting and uneven paint sorption caused by severe vertical density variations across the panel surface.
As indicated above, applications of layers of fines in the form of wood fibers or particles have been tried in an effort to solve the aforementioned problems. This technique reduces the wafer telegraphing with moisture. However, the surface does not accept liquid finish well due to grain raising. Also, vertical density variations are inescapable, resulting in variable sorption of the liquid finish across the surface.
Resin-impregnated paper surfaces can be adhered to the surface of lignocellulosic products during the pressing step in an effort to provide finishable surfaces which somewhat reduce telegraphing. Such surfaces may be smooth as formed but may telegraph the wafer or strand shape on exposure to water and/or high humidity. Performance may be improved by first sanding the panel product and thereafter applying the paper to the sanded panel in a separate step. However, it has now been determined that these types of products do not meet the requirements for producing smooth multi-layer panels of the type described above with respect to retention of smoothness appearance under moderate to severe moisture exposure conditions. Furthermore, this approach requires higher cost multiple handling steps as compared to the present invention employs a lower cost, integrated method which is typically performed in a one-step process.
More specifically, a method is provided for forming a durable, smooth sided, multi-layer integral, composite engineered panel including outer layers having first and second major outer surfaces. The method typically is directed to first forming a mat comprising a plurality of layers of lignocellulosic strands or wafers. The mat includes outer layers having first and second major outer surfaces.
The plurality of layers of lignocellulosic strands or wafers are bonded together by an adhesive material. The amount of adhesive by which the lignocellulosic strands or wafers are bonded together is preferably at least about 2% by weight, more preferably, at least about 3% by weight, most preferably, at least about 4% by weight, based on the weight of the mat.
Preferably, the adhesive bonding material in the mat is a phenol-formaldehyde resin and/or an isocyanate resin. Typically, the isocyanate resin employed is a diisocyanate resin.
Then, at least one layer of smaller particles of lignocellulosic material, including outer layers having first and second major outer surfaces, is formed on the mat. This layer of smaller particles is blended together with an adhesive material thereby joining the layer of smaller particles to at least one of the first and second major outer surfaces of the mat. The amount of the adhesive material employed is preferably at least about 4% by weight, based on the weight of the smaller particles. The average size of the smaller particles is less than the average size of the particles of lignocellulosic material which comprise the mat. The smaller particles preferably have an average particle size of not more than about 2 mm, and more preferably an average particle size of not more than about 1 mm.
As previously described above, certain prior art inventions require that the fines be xe2x80x9cgradedxe2x80x9d for uniform size, and that the distribution of these graded fines be specific. Contrarily, in the present invention, grading is optional. Thus, the subject method permits the smaller particles employed in the present invention to be selected from a group consisting of graded particles and ungraded particles, not merely formed of selectively graded particles as required by a number of prior art methods.
The panel preferably comprises from at least about 60% by weight up to about 95% by weight of the lignocellulosic strands or wafers in the mat, and from about 40% by weight to about 5% by weight of the small particles, more preferably comprises from at least about 65% by weight up to about 90% by weight of the lignocellulosic strands or wafers in the mat, and from about 35% by weight to about 10% by weight of the small particles.
Next, at least one sheet of flexible material, having first and second major outer surfaces, is joined to at least one of the first and second major outer surfaces of an outer layer of smaller particles of lignocellulosic material. This step takes place during the formation of the multi-layer integral, composite engineered panel. The sheet is typically impregnated with an adhesive material prior to joining the sheet to at least one of the first and second major outer surfaces of the mat to form the subject integral engineered panel.
Preferably, a sheet of paper is employed as the flexible material. The resin-impregnated sheet preferably has a paper basis weight of at least about 25 lbs. per ream, more preferably at least about 35 lbs. per ream, and most preferably at least about 50 lbs. per ream. The sheet also preferably has a weight of at least about 0.015 lbs./sq.ft., more preferably a weight of at least about 0.02 lbs./sq.ft., and most preferably a weight of at least about 0.03 lbs./sq.ft. The paper can also be pre-primed on its outer surface to further facilitate paintability.
Finally, a substantially smooth outer surface is formed, upon pressing, in at least one of the first and second major surfaces of the multi-layer integral, composite engineered panel. The substantially smooth state of that surface is maintained even when the multi-layer integral, composite engineered panel is exposed to water and/or high humidity conditions. Also, wafer/strand telegraphing is substantially eliminated so that the smooth outer surface is substantially devoid of that undesirable condition.
The panel of this invention is typically paintable. Moreover, when the step of painting the smooth outer surface is conducted, the resultant painted panel is produced without substantial grain raising thereof or wafer/strand telegraphing. In the manufacture of the above-described smooth-surface product of the present invention, there can also be a problem of xe2x80x9ccuppingxe2x80x9d when a panel exits the press section. The presence of cupping creates an unsightly product that is difficult to market. The mats of this invention are formed of layers of strands or wafers, overlaid first with a layer of smaller particles, and having an outer layer which is a sheet of flexible material, typically paper, which is preferably impregnated with a bonding material. Cupping is likely to occur with any unbalanced construction such as with the panel of the present invention. Prior art methods, which include rebalancing the orientations of the various layers, have done little to reduce cupping.
It has now also been discovered that cupping of the smooth surface multi-layer lignocellulosic product of the present invention can be substantially eliminated by moisturizing the subject product. This invention encompasses a process for producing non-cupped board. In normal manufacture, non-cupped board is not attainable out of press. Boards exit the manufacturing process typically at less than 1% moisture and, after unitizing and strapping, the rate of moisture absorption is extremely slow. Although boards are strapped flat, there is little or no stress relaxation because so little moisture is absorbed into the stack. Thus, in our just-in-time economy, products may be unstrapped on the store""s shelves within a matter of days or weeks, and therefore cupping will spring back in the board.
The panel is maintained at a moisture level sufficient to prevent cupping of the panel. The moisture level in the panel is preferably maintained at a level of at least about 3% by weight, more preferably at a level of at least about 4% by weight, and most preferably at a level of at least about 5% by weight, based on the weight of the panel. One way of accomplishing the formation of a non cupping panel is by exposing the panel, after formation, to high humidity. Another way to prevent cupping of the panel is by post press wetting of the panel, after formation, with a liquid wetting agent. A third approach to prevent cupping of the panel is by increasing the moisture level of the lignocellulosic particles in the mat. Moisture can also be added to prevent cupping of the panel by spraying moisture onto at least one of a plurality of locations in the mat during formation.